Magnetic resonance imaging (MRI) techniques have been in commercial use since the 1970s. The most commonly deployed MRI setups involve three magnetic field generators: a static uniform magnetic field, a gradient magnetic field system (three gradient fields required for 3-D imaging), and a radio-frequency (RF) transmitter, as well as one or more magnetic field detectors, in the form of an RF detector. Each of these is an expensive item in a whole body MRI. If one sub-system could be eliminated, then costs would be substantially reduced, and costs of efficiently providing the remaining fields can be made more efficient, and spatially compact.
The cost of producing these fields imparts high cost to Magnetic Resonance Imaging (MRI). Access to MRI and long waiting lists is an important issue for health-care world-wide. There is no effective low-cost imaging modality for imaging the central nervous system (CNS). The gradient pulse DB/dt (gradient pulse) is also limited by FDA regulations.
Lower cost MRI could open-up new application areas and markets.
Other RF Encoding Methods in MRI
The idea of using a RF transmitter to encode k-space (albeit in a different fashion) using amplitude gradient B1 fields is known in the art. For example, numerous recent patents and papers are devoted to Transmit SENSE type apparatus. Typically Transmit SENSE type apparatus do not remove the gradient magnetic field systems.
Rotating Frame imaging (rotating frame zeugmatography) (D. I. Hoult, J. Magn. Reson. 33, 183 (1979)) is an earlier method that attempts to form magnetic resonance images by use of magnet and RF coils only. This method has not been successful as a clinical imaging method however, principally because it requires very large flip angle RF pulses, which, for large samples, requires high RF power. The method has been used successfully in some smaller-scale applications, usually 1D experiments with surface coils, (J. Magn. Reson 60, 268-279 (1984), Michael Garwood, Thomas Schleich, Gerald B. Matson Spatial Localization Of Tissue Metabolites By Phosphorus-31 NMR Rotating-Frame Zeugmatography)
Another approach to RF-based localization, which is analogous to slice-selection, is taught by Bendall (J. Magn. Reson. 53,365-385 (1983), M. Robin Bendall, Roy E. Gordon, Depth And Refocusing Pulses Designed For Multipulse NMR With Surface Coils.). This method requires multiple excitations with phase-cycling to achieve selective excitation.
Related methods, using a gradient in the amplitude of the RF field and applied to micro-imaging only are taught, for example, in: P. Maffei, J. Magn. Reson. Series A 107, 40-49 (1994) NMR Microscopy by Radiofrequency Field Gradients; and F. Humbert, J. Magn. Reson. Series A, 123 242-245 (1996) NMR Microscopy by strong RF Gradients
All of these approaches require RF fields with amplitude gradients.